Historically, metal forming has involved forging, stamping, drawing, welding, and bending processes, just to name a few for illustrative purposes. In recent years, hydroforming has been shown to be useful in imparting complex shapes to a metal blank, whether tubular or sheet. In a typical hydroforming operation, a workpiece blank is cut and preformed into the approximate shape of the finished product, if the shape is conducive to the preforming step. The blank is then placed in a cavity formed within a typical two-part split die. A hydraulic press closes the die and applies a die-closing pressure, the amount of which is determined in part by the component geometry and the forming parameters.
If the blank is tubular, the ends thereof are sealed by means of hydraulic rams and the tube interior is filled with a fluid, usually an incompressible fluid, and typically a water and oil mixture. Increasing the fluid pressure in the blank causes it to yield and plastically conform its outer surface expansively into the shape of the interior surface of the die cavity. This plastic expansion displaces the gas initially present in the cavity space between the blank and the die. Because the fluid is usually incompressible, the increased pressure is usually achieved by feeding additional fluid into the blank. Once the piece is conformed to the shape of the die cavity, the tube ends are unsealed, the forming fluid is depressurized and drained, and the press and die opened to remove the formed component.
If the blank is a sheet, a periphery of the sheet may be sealed between die halves by the hydraulic press and a space between one half of the die and the sheet may be pressurized with the fluid. Increasing the fluid pressure in the blank causes it to yield and plastically conform its outer surface expansively into the shape of the interior surface of the other die half cavity. This plastic expansion displaces the gas initially present in the cavity space between the sheet and that other die half. Because the fluid is usually incompressible, the increased pressure is usually achieved by feeding additional fluid into the space. Once the piece is conformed to the shape of the die cavity, the forming fluid is depressurized and drained, and the press and die opened to remove the formed component.
In some circumstances, the blank can be a pair of sheets, with a small spacing left between them when they are registered in the press. A periphery of the registered sheets may be sealed against each other or an interstitial sealing surface by the hydraulic press. The space between sheets may then be pressurized with the fluid. Increasing the fluid pressure in the blank causes each of the sheets to yield and plastically conform its outer surface expansively into the shape of the interior surface of its respective die cavity. This plastic expansion displaces the gas initially present in the cavity spaces between the respective sheets and facing die halves. Because the fluid is usually incompressible, the increased pressure is usually achieved by feeding additional fluid into the spacing. Once the sheets are conformed to the shape of the die cavity, the forming fluid is depressurized and drained and the press opened to remove the formed component sheets.
Much of the prior attention in hydroforming technology has been directed at steel and other ferrous alloys. However, the automotive industry's interest in improving fuel efficiency by reducing vehicle weight has caused a desire to shift in the materials selected for automobile fabrication from plain carbon steels, to higher strength-to-weight materials such as high strength steels and some high-performance polymers. Lightweight aluminum alloys are also being implemented increasingly into vehicle structural components. In the future, even higher strength-to-weight materials, such as magnesium, will become attractive if the formability of these materials can be improved.
Unfortunately, lightweight materials, such as aluminum and magnesium, typically exhibit relatively low formability at room temperatures, although their formability at higher temperatures is quite acceptable. These materials also typically exhibit sensitivity during forming to strain-rates, a sensitivity that is further affected by temperature. For these reasons, the rate of forming and forming temperature of the workpiece must be controlled. Temperature gradients in the workpiece, while desirable in some situations, may complicate the forming process. When hydroforming is conducted at temperatures significantly different from ambient, contact of the workpiece with air or with solid surfaces that are at ambient temperatures may result in uneven heating or cooling of the workpiece, even during the hydroforming operation.
To form materials at elevated temperatures, a workpiece blank must be preheated to the proper forming temperature to enhance formability. The temperature distribution within the part can have significant effect on the finished part quality since an uneven temperature distribution will correspondingly produce uneven levels of formability, which can produce an undesirable wall-thinning distribution within the part. Consequently, it is generally desirable to maintain a uniform temperature distribution during part forming. Attempts have been made to use a gas as a pressurizing fluid medium to hydroform a part, but one major drawback in using a gas lies in its compressibility as well as sensitivity to volumetric change imparted by temperature change making pressure and volume control of the fluid difficult during the forming process. To control material strain-rate, an incompressible fluid is ideally suited to allow precise volume control of the pressurizing medium during the forming process. As compared to a gas, a liquid can be considered to be essentially incompressible and provides a superior medium by which to hydroform parts. If parts are to be formed using a liquid medium, that medium should preferentially, for example, be capable of working in the part forming temperature range without boiling, be resistant to excessive oxidation, and be nonflammable. Consequently, new methods are needed to hydroform materials at elevated temperatures using liquid media. Additionally, current hydroforming processes operating at ambient temperatures that use liquid media require pumping systems and fluid plumbing loops to fill and empty the workpiece or forming apparatus before and after the forming process. Another disadvantage of current hydroforming methods requires a separate step of applying lubricants to the workpiece before hydroforming.
It is, therefore, an advantage of the present invention to provide a hydroforming device and method where the temperature of the workpiece is controlled in an immersion bath of fluid, preferably the hydroforming fluid, at least before and during the hydroforming process. Another advantage of the present invention is appreciated by providing a more effective and efficient method of introducing and removing the pressurizing liquid medium to and from a workpiece during a hydroforming operation. Yet another advantage of the present invention is appreciated by providing a more effective and efficient method of introducing lubricants to a workpiece. Further advantages of the present invention will be described subsequently in more detail herein.